1. Field of the Invention
The present invention relates to a film formation method and a film formation device for forming, on a substrate, a film containing constituent elements of a target through vapor deposition using plasma. The present invention also relates to a piezoelectric film formed with the film formation method, as well as a piezoelectric device, a liquid discharge device and a piezoelectric ultrasonic transducer including the piezoelectric film.
2. Description of the Related Art
Sputtering is a film formation method, in which a substrate and a target are placed to face each other, and a gas which is plasmized under reduced pressure is made to collide against the target, so that the energy of the collision makes molecules or atoms be ejected from the target and deposited on the substrate. In vapor phase film formation, the substrate is usually an insulator, and is electrically insulated from the ground, i.e., is floated.
U.S. Pat. No. 5,510,011 (hereinafter referred to as patent document 1) has proposed to carry out sputtering with applying a predetermined bias potential to the substrate to increase incident energy of positive ions hitting the substrate.
Patent document 1 has also proposed to provide a third electrode that surrounds a plasma discharge space formed between a sputter electrode, which is connected to a radio frequency power supply for generating plasma, and a substrate electrode, which is connected to a DC power supply or a radio frequency power supply for applying a positive bias, and to place a target material on the third electrode and apply a negative DC voltage to the third electrode (claim 1).
Patent document 1 has further proposed: to connect to a substrate electrode a DC power supply for applying a biasing DC voltage; to connect to a sputter electrode a radio frequency power supply and a DC power supply; to alternately apply, with an alternate applying means, a DC voltage which is lower than a threshold value for effecting sputtering of a target material and a DC voltage which is higher than the threshold value; and to vary, with a matching circuit control means, a circuit constant at a matching circuit (matching box) of the radio frequency power supply synchronously with the changes of the DC voltage (claim 4).
Patent document 1 has further proposed: to connect to a sputter electrode a radio frequency power supply; to connect to a substrate electrode a DC power supply for applying a DC voltage; to detect, with a floating potential detection means, a floating potential in a plasma discharge space, or to detect, with a radio frequency current detection means disposed between the substrate electrode and the DC power supply, a current value flowing through the substrate; and to control, with a substrate potential control means, the DC voltage applied to the substrate electrode based on the detected floating potential or current value (claim 5).
Japanese Unexamined Patent Publication No. 2002-129320 (hereinafter referred to as patent document 2) has proposed to carry out sputtering with controlling the substrate potential to be substantially zero in order to prevent electrostatic breakdown of the thin film being sputtered.
Patent document 2 has further proposed to provide a control electrode at a position lateral to a sputter space formed between a target, which is connected to a sputter power supply for applying a negative voltage to generate plasma, and a substrate on a wafer stage, which is connected to a bias power supply which is an AC power supply for applying a desired biasing voltage (for example, a negative voltage or an AC voltage), and to carry out sputtering with the substrate being floated and applying to the control electrode a control voltage which makes the potential on the substrate be substantially zero volt (claim 1).
Theoretically, the composition of a film formed through sputtering should be substantially the same as the composition of a target. However, if the constituent elements of the film include an element with high vapor pressure, the element tends to be subject to reverse sputtering on the surface of the formed film, and this may often make it difficult to provide the film with substantially the same composition as the composition of the target.
The reverse sputtering is a phenomenon in which, if there is a large difference in ease of sputtering (sputter rate) among the constituent elements, one of the constituent elements which is more susceptible to sputtering than the other elements deposited on the substrate is preferentially sputtered out of the surface of the film, although the elements sputtered from the target have almost equal composition as the target.
For example, in PZT (lead zirconate titanate), which is a perovskite oxide with high ferroelectricity, or in a substitution system thereof, Pb is more susceptible to the reverse sputtering than Ti and Zr, and thus the Pb concentration in the film tends to be lower than the Pb concentration in the target. Also, in a perovskite oxide containing Bi or Ba at the A-site, these elements have high vapor pressure and thus have the similar tendency.
In Zn-containing compounds, Zn has high vapor pressure and thus has the similar tendency. For example, in a zinc oxide transparent conductive film or transparent semiconductor film, such as InGaZnO4 (IGZO), which has excellent electric and optical characteristics comparable to indium tin oxide (ITO) and which is inexpensive and is an abundant resource, Zn is more susceptible to the reverse sputtering than the other constituent elements, and the Zn contents in the film composition tends to be lower than that in the target composition.
For the systems as presented in the above examples, in order to obtain a desired composition, countermeasures, such as using a target which contains the element susceptible to the reverse sputtering with an increased concentration, have been taken.
The present inventors have formed piezoelectric films of PZT or a substitution system thereof on substrates with 6-inch or more diameter using a commercially available sputtering device, and have found that the Pb concentration of the film varies in the in-plane direction. With a composition which is susceptible to the reverse sputtering, it is considered to be necessary to more strictly control film formation conditions to provide homogeneous conditions in the in-plane direction.
Patent documents 1 and 2 both pertain to sputtering film formation in general, and they do not mention piezoelectric films or Zn-containing compound films. Further, they provide no particular advantage to achieve homogenized film composition regardless of the composition of the film.
The above-described problem is not limited to the case of sputtering. Similar problem may occur in other film formation methods in which a substrate and a target are placed to face each other and a film containing the constituent elements of the target is formed on the substrate through a vapor-phase growth process using plasma. This problem is more apparent when the substrate size is larger, such as a substrate with 6-inch or more diameter.